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Journal of Magnetism and Magnetic Materials 310 (2007) 2702–2704 www.elsevier.com/locate/jmmm
Magneto-optical properties in diluted magnetic semiconductors Cd0.65yMn0.35NiyTe single crystals Y.H. Hwanga, H.K. Kima, S. Choa, Y.H. Uma,, H.Y. Parkb a b
Department of Physics, University of Ulsan, Ulsan 680-749, Republic of Korea Semiconductors Applications, Ulsan College, Ulsan 680-749, Republic of Korea Available online 20 November 2006
Abstract We investigated the magneto-optical properties of diluted magnetic semiconductor Cd0.65yMn0.35NiyTe single crystals grown using a vertical Bridgman method. This material crystallizes in the zinc-blende structure for values of yo0.06. The fundamental enegy gap was increased and the lattice constant was decreased showing inverse relationship with the band gap energy with increasing Ni contents. The Verdet constant increased with increasing y, which is consistent with behavior of the magnetization. The Faraday rotation in Cd0.65yMn0.35NiyTe crystals was increased as Ni contents was increased, which is due to the increasing of magnetization with Ni contents. r 2006 Published by Elsevier B.V. PACS: 73.61.Ga Keywords: DMS; Cd0.65yMn0.35NiyTe; Faraday rotation; Bridgman method
Diluted magnetic semiconductors (DMS) are based on II–VI semiconductor compounds in which a fraction of the nonmagnetic cations is randomly replaced by magnetic ions [1]. One of the most striking effects observed in DMS is the giant Faraday rotation at photon energy close to the band-gap energy. This property makes DMS promising candidates for fabricating magneto-optical devices, such as magnetic field sensors, isolators, and modulators [2]. The Faraday rotation in II–VI DMS has proved to be quite large due to the giant Zeeman splitting of the exciton levels. In particular, for Cd1xMnxTe, a large exciton Zeeman splitting is observed, and this gives rise to a giant Faraday rotation at room temperature. Recently, it is reported that the Faraday rotation can be enhanced by an addition of different magnetic ions to CdMnTe [3]. In this work, we investigated the magneto-optical properties of Cd0.65yMn0.35NiyTe single crystals, and showed the variation of the Faraday rotation as a function of temperature with various Ni composition y.
Corresponding author. Tel.: +82 52 259 2327; fax: +82 52 259 1693.
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[email protected] (Y.H. Um). 0304-8853/$ - see front matter r 2006 Published by Elsevier B.V. doi:10.1016/j.jmmm.2006.10.999
The vertical Bridgman method was used for the growth of Cd0.65yMn0.35NiyTe single crystals. The composition and homogeneity of the grown samples were evaluated by EPMA and the XRD experiment was carried out to examine the crystal structure and the lattice constant of the Cd0.65yMn0.35NiyTe crystal. The Faraday rotation was measured by using Glan-prism polarizers at 10–300 K. Fig. 1 shows the XRD spectrum for Cd0.6Mn0.35Ni0.05Te, which shows that the crystal structure is zinc blended, and the inset shows the lattice constants obtained from the XRD spectra for the samples with various Ni compositions up to y ¼ 0.05. The lattice constants decreased as Ni composition increased, which is attributed that the covalent radii of Mn and Ni are smaller than that of Cd. In addition, the near band-edge emission obtained from the photoluminescence spectra is linearly increased with the addition of Ni into CdMnTe (see Fig. 2(a)). There are three transitions, which is a characteristic of the Cd0.65yMn0.35NiyTe crystal: peak A, corresponding to the excitonic recombination across the fundamental energy gap, peak B, corresponding to transition from the first excited level 4G to the ground state 6S of the localized Mn2+ ions, and peak C, correlated with the d–d transitions of the tetrahedrally coordinated Ni2+ ions.
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The energy gaps of Cd0.65yMn0.35NiyTe for y ¼ 0, 0.01, 0.03, and 0.05 were measured to be 2.036, 2.044, 2.05, and 2.067 eV, respectively, at 200 K. An increase of the energy gap with increasing Ni ion may be due to a decrease in lattice constant. Also, energy gap of the quaternary Cd0.65yMn0.35NiyTe crystal decreases with increasing temperature as shown in Fig. 2(b). We measured the temperature dependence of magnetization (M) for the Cd0.65yMn0.35NiyTe crystals as shown in Fig. 3. Magnetization increases with increasing Ni contents. This indicates that the spin–spin interaction should become enhanced by the addition of Ni. Fig. 4 shows the Verdet dispersion curves at 10 K for Cd0.65yMn0.35NiyTe with Ni contents up to y ¼ 0.05. In wide-gap DMS such as CdMnTe crystals, the Faraday rotation is very large in general with a sign opposite Fig. 1. (a) y2y XRD spectrum for the powdered Cd0.6Mn0.35Ni0.05Te indicates a zinc-blended structure.
Fig. 3. Temperature-dependent magnetization (M) for Cd0.65yMn0.35 NiyTe crystals between the 5 and 350 K at 500 Oe magnetic field. The open symbol and closed symbols indicated zero-field and field-cooled data.
Fig. 2. (a) PL spectra of the Cd0.65yMn0.35NiyTe with y ¼ 0.01, 0.03, and 0.05 at 12 K. (b) Temperature dependence of the PL peak A for Cd0.65yMn0.35NiyTe. Solid lines show fits to the Varshni relation [4].
Fig. 4. Verdet constant as a function of photon energy for Cd0.65yMn0.35 NiyTe with various Ni contents y at 10 K. The points are experimental data, and the solid lines are the best fit using the single oscillator model [5].
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to that in a nonmagnetic semiconductor of CdTe. We observed the Faraday rotation for Cd0.65yMn0.35NiyTe with Ni contents and the Faraday rotation was increased sequentially since the Faraday rotation is proportional to the magnetization [5]. These values are consistent with the results of Ahn et al. [6]. In conclusion, the Cd0.65yMn0.35NiyTe crystals with various Ni mole fractions were grown in the zinc-blended structure and band-gap energy was increased and the lattice constant was decreased showing inverse relationship with the band-gap energy with increasing Ni contents. The Faraday rotation was increased as Ni contents were increased and the origin of the enhancement of the Faraday rotation in Cd0.65yMn0.35NiyTe crystals can be explained in terms of the magnetization. The Verdet constants of the Faraday rotation for Cd0.65yMn0.35NiyTe
with y ¼ 0.0, 0.01, 0.03, and 0.05 were measured to be 0.9, 1.15, 1.27, and 1.501/G cm, respectively, at 614 nm (T ¼ 10 K). This work was supported by 2004 Research Fund of University of Ulsan. References [1] J.K. Furdyna, J. Appl. Phys. 53 (1982) 7637. [2] N. Kullendorff, B. Hok, Appl. Phys. Lett. 46 (1985) 1016. [3] J.Y. Ahn, M. Tanaka, M. Imamura, J. Magn. Magn. Mater. (2001) 226–230. [4] Y.P. Varshni, Physica 34 (1967) 149. [5] D.U. Bartholomew, J.K. Furdyna, A.K. Ramdas, Phys. Rev. B 34 (1986) 6943. [6] J.Y. Ahn, M. Tanaka, M. Imamura, J. Appl. Phys. 89 (2001) 7395.
